Wild Things

The skin of the Asian common toad is laced with a deadly toxin. A new study finds that most predators in Madagascar that might eat the toad lack the genetic mutations necessary to make them immune to the toxin.

At some point eight to 10 years ago, some toads stowed away on a ship in Asia, possibly Ho Chi Minh City, and hitched a ride to Madagascar. Those invaders, Asian common toads, have been slowly spreading across the large island ever since.

The toad’s skin contains a toxin that kills nearly anything that tries to eat the amphibian. Scientists have been warning of the toad’s danger to ecosystems for years, but they’ve lacked evidence of just how dangerous the toads could be. Now, a genetic study confirms that nearly all of Madagascar’s predators would be vulnerable to the toad’s toxin.

Duttaphrynus melanostictus is one of many toad species that secrete potent toxins called bufadienolides. These chemicals disrupt the flow of sodium and potassium in cell walls, something that is particularly important for the function of muscles, and especially the heart.

“Animals that are not resistant to the toads that take a mouthful of toad can die extremely quickly from heart failure,” says Wolfgang Wüster, a herpetologist at Bangor University in Wales.

There are species, including reptiles and mammals, that have evolved resistance to the toxin. And in 2015, Wüster and other scientists reported that these examples of resistance shared a commonality: They all had specific mutations in the gene for the sodium-potassium pump. “That’s a universal mechanism for being able to consume toads — and particularly to deal with the bufo toxins,” says Wüster.

Toxic toads are an invasive species in several places around the world. Cane toads, for instance, have been spreading across northern Australia for decades, leading to declines of species such as quolls and snakes. That invasion is well-studied, and so scientists know a lot about the species affected by it. There’s little need there to start looking for species that are immune to the toxin. But in areas of more-recent invasions, such as Indonesia and Madagascar, such data could be helpful.

So Wüster and his colleagues looked at the gene for the sodium-potassium pump in 77 Malagasy species, including 28 birds, eight mammals and 27 snakes. Only one species, the white-tailed antsangy, a type of rodent, had the mutations that would make it immune to the toad toxin. The mutations were the same as those found in the brown rat, which suggests that they might be a common feature among rodents. The 76 other species studied are likely vulnerable to the toad toxin, the researchers conclude June 4 in Current Biology.

This doesn’t mean, however, that Madagascar’s wildlife is entirely doomed. In Australia, scientists have documented behavioral changes in some predator species that have allowed them to safely eat toads. Some birds, for instance, have learned that toad tongues are safe to eat. Some Malagasy species could likewise figure out how to safely make a meal of the toads — or learn to not eat them at all.

People might help as well. It’s probably too late to eradicate the toads from Madagascar. There’s too many, and there’s no good way to easily find and kill them. But this research may help scientists in Madagascar to take measures to protect certain species or sites from the invasive species, Wüster says.

“Madagascar has been isolated for tens of millions of years,” notes Wüster. “It’s never had toads. Why would the local animals be resistant to toads? We always expected them to be sensitive. But now we have much better evidence that that’s really the case.”